Refactor completions (#41939)

This is progress toward multi-word snippets (including snippets with
prefixes containing symbols)

Release Notes:

- Removed `trigger` argument in `ShowCompletions` command

---------

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

crates/editor/src/code_context_menus.rs

@@ -252,8 +252,17 @@ enum MarkdownCacheKey {
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 pub enum CompletionsMenuSource {
+    /// Show all completions (words, snippets, LSP)
     Normal,
+    /// Show only snippets (not words or LSP)
+    ///
+    /// Used after typing a non-word character
+    SnippetsOnly,
+    /// Tab stops within a snippet that have a predefined finite set of choices
     SnippetChoices,
+    /// Show only words (not snippets or LSP)
+    ///
+    /// Used when word completions are explicitly triggered
     Words { ignore_threshold: bool },
 }